1. Field of the Invention
The present invention relates to a radio communication system capable of urging a base station to execute adaptive control of the modulation scheme and the error-correcting-coding rate, for example, on the basis of CQI (Channel Quality Indication) which is transmitted from a mobile station to the base station.
2. Description of the Related Art
In a conventional system employing the adaptive modulation, the mobile station measures radio transmission path quality of a base station having a most desirable receiving quality, determines a receivable transmission format (i.e. combination of the modulation and the coding rate) on the basis of the measured value of the radio transmission path quality, and transmits the transmission format to the base station as CQI (Channel Quality Indication).
On the basis of CQI, the base station transmits transmission information over an individual information channel, by changing the combination of the modulation scheme of the data to be transmitted to the mobile station and the error-correcting-coding rate. In other words, when the receiving condition of the mobile station is preferable, the base station executes the transmission at a high-speed data transmission rate having a low error tolerance. When the condition of the radio waves is poor, the base station executes the transmission at a low-speed data transmission rate having a high error tolerance.
The data communications are thus made at various communication rates corresponding to the downstream receiving quality, but the determination of the downstream receiving quality is based on a table prestored in the mobile station. This table directly represents a predicted downstream data communication speed, which is a very correct data communication speed obtained by considering the correction using statistic data such as the error rates of predicted and previous downstream data transmissions.
The radio system which implements high-speed data communication by executing such adaptive modulation is generally in a best-effort type service form. For this reason, the mobile station requires the only base station of the most desirable receiving quality to make communications. The base station transmits packets, with priority, to the mobile station which requires the high transmission rate of preferable downstream receiving quality (Maximum CIR).
For this reason, the priority in making communications with the base station is low for a mobile station which does not have preferable downstream receiving quality. As a method of solving this and increasing both the user throughput and the base station throughput in good balance, for example, PF (Proportional Fairness) scheduler method of the 1xEV-DO (Evolution-Data Optimized) system is known (refer to, for example, IEEE International Conference, VTC, 2000 Spring, “Data throughput of CDMA-HDR a High Efficiency-High Data Rate Personal Communication Wireless System” written by A. Japali, R. Padovani and R. Pankaj).
The PF scheduler method employs evaluation function values calculated at “SNR_inst/SNR_ave” as an index which the base station uses for user selection. The base station calculates the evaluation functions values for the respective mobile stations and selects the mobile station which has the greatest evaluation function values. “SNR_inst” represents instantaneous SNR (Signal to Noise Ratio) of which the mobile station notifies the base station. “SNR_ave” represents an average value of SNR.
In the OFDM (Orthogonal Frequency Division Multiplexing) system, communications are made by simultaneously employing a number of subcarriers. At this time, the mobile stations can be assigned to the subcarriers, respectively. The OFDM system is similar to the conventional FDM system with respect to assigning a specific frequency to each of the mobile stations. However, the OFDM system is significantly different in the point that each mobile station receives all the subcarriers of the OFDM simultaneously, executes reception of the OFDM signals, and takes the subcarrier which it should receive.
In a system such as the OFDM system in which a plurality of resource blocks can be arranged in a direction of frequency, for a unit time, however, the amount of CQI information transmitted in response to the number of resource blocks in the frequency direction is increased as compared with the system such as the conventional EV-DO, HSDPA system in which there is only one resource block for the unit time. Therefore, deterioration of the throughput of the upstream link becomes more significant.